The GRAAL (Gamma-Ray
Astronomy
at ALmeria) project is presently using the CESA-I heliostat field
at the Plataforma Solar de
Almeria (PSA) as a gamma-ray telescope with an energy threshold of
around 250 GeV. CESA-I consists of 300 heliostats with a mirror size
of 39.7 m2 each which reflect the Sun light onto a central tower.
During the night,
63 heliostats direct
the Cherenkov light generated by air-showers arriving in the direction
of the cosmic source, through four non-imaging light concentrators housed
in a special enclosure in the tower
which collect the Cherenkov light into a large-area PMT each. The incoming
Cherenkov light consists of a train of pulses
each one coming from a different heliostat. The arrival time and amplitude
of the pulses is measured by using fast electronics.
Figure 1a): The CESA-I heliostat field at the Plataforma Solar de Almeria during the day.
Figure 1b): CESA-I is used at night for the detection of the Cherenkov
light produced by extensive air showers generated by cosmic gamma-rays
(artistic view).
The GRAAL project is a collaboration of the following institutions:
